Flywheel

ABSTRACT

An apparatus and method for transmitting torque from a flywheel inside a vacuum chamber to a driveshaft outside the vacuum chamber includes constructing a magnetic gear coupling using a pair of permanent magnet arrays having magnetic flux coupling elements arranged in the vacuum chamber wall therebetween so as to effectively couple magnetic flux between the arrays.

The invention relates to a flywheel and a method for constructing aflywheel for energy storage.

BACKGROUND OF THE INVENTION

Flywheels are known for the storage of energy in the form of kineticenergy, for example for use in vehicles. In such instances it is knownto use a flywheel to store the energy which would otherwise be convertedto heat in the vehicle's braking system when the vehicle decelerates,this stored energy then being available for use to accelerate thevehicle when desired.

An existing type of flywheel according to FIG. 1 has a central metallicsupport section (1) which can be mounted on a central support such as ashaft. At least one composite ring (2) is mounted on the central supportsection. The composite ring in this type of flywheel is filament woundfrom carbon fibre. When the flywheel is in rotation, the ring will tendto expand in diameter due to the centrifugal forces acting on it. Thering has high strength in hoop for re-acting the centrifugal forces whenthe flywheel is in rotation. However, the outer ring can become a loosefit on the central support section and potentially (dangerously) becomedismounted from the central support section. In addition the radialstress can result in failure of the composite ring.

In order to counteract the tendency of the ring to grow, the ring istypically machined with a smaller inner diameter than the outer diameterof the central support section and is then mounted onto the centralsupport section with an interference fit. The mismatch in diametersresults in a pre-load such that that ring exerts an inward force ontothe central support section. This inward preload is greatest when theflywheel is not rotating and results in a requirement for the centralsupport section to be sufficiently structurally strong that it canwithstand the preload force when the flywheel is stationary. It is knownfor more than one composite ring to be pressed together and furthermounted onto the central support. The pre-load increases towards thecentre of the flywheel and with the number of rings pressed together.Consequently a large amount of material may be required in the centralsupport section of the flywheel in order to counteract this pre-loadforce, and this material, being near the centre of the flywheel, addsonly very inefficiently to the rotational inertia of the flywheel.Further, if the hub is stiffer than the composite ring, as the speed ofthe flywheel increases and the pre-load reduces then the increased masswill lead to stress management problems in the hub.

Yet further, in the existing system, exceeding the maximum stress ratingof the composite ring will result in failure. In the flywheel typeabove, the central support section exerts an outward force on thecomposite ring due to the pre-load. This force is in the same directionas the centrifugal forces acting on the ring when the flywheel is inrotation. Then, if the stiffness of the hub is lower than the compositering, the ring must be strong enough to counteract the sum of thepreload force and the centrifugal forces when the flywheel is rotatingat maximum speed. A further problem with this type of flywheel istherefore that the preload reduces the maximum rotation speed of theflywheel.

A further problem with existing systems is that if a flywheel is to becoupled to, for example, a vehicle transmission, a splined coupling isnormally required in order that high transient torque levels (forexample when the vehicle gearbox ratio is changed quickly, thusrequiring the flywheel to accelerate or decelerate rapidly) may betransmitted to the flywheel without slippage.

A flywheel of the type described in UK patent application 0723996.5,filing date 7 Dec. 2007, overcomes the aforementioned limitations byproviding a flywheel having a drive transfer element and a rimcomprising a mass element, where the rim and the drive transfer elementare coupled by a winding. However, it is desirable with this type offlywheel to have an indication of stress in the flywheel components asthe flywheel is rotated at increasing speed.

UK patent application 0902840.8 provides such an aforementionedindication of stress in the flywheel components by incorporating awarning, or indicator, ring into the flywheel. The indicator ring can bemounted to the flywheel with an interference fit, such that residualstresses are set up between the ring and the flywheel. The level ofinterference fit, or preload, and the relative stiffnesses of the ringand the part of the flywheel onto which the ring is mounted, are chosensuch that when the flywheel is rotated at or in excess of apre-determined trigger speed, the preload is substantially overcome bycentrefugal forces, causing the ring and support member to at leastpartially separate. The ring is then able to move on the flywheel,causing an “out of balance” condition, resulting in a vibration which isdetectable as an indication of stress in the flywheel components.

A further problem with existing flywheels is the need to finely balancethe rotating mass of the flywheel. Since the kinetic energy stored in arotating flywheel is proportional to {acute over (ω)}² (where {acuteover (ω)} is the angular velocity of the flywheel), increasing themaximum rotational speed of a flywheel allows more energy to be storedin a flywheel of a given mass, and thereby increases the energy storagedensity of such a flywheel. However, as the rotational speed increases,the balance of the assembly becomes more critical, as does proving thestructural integrity of the flywheel. Furthermore, the cost of balancinga flywheel generally increases with the level of accuracy of balancerequired.

A further problem when balancing composite flywheels, such as the typedescribed in UK patent applications 0723996.5 and 0902840.8, is thatonly a limited amount of machining/processing can be performed on thecomposite component (i.e. the mass bearing rim) without severelyaffecting the structural integrity of the composite. This therebyaffects the simplicity of the balancing process, since material has tobe removed from the flywheel at a location away from the composite rim.

A further problem is that existing methods for balancing flywheelsgenerally incorporate machining and/or grinding and/or drilling ofmaterial from the flywheel. Not only can (as previously mentioned) suchmachining and flash or grinding and/or drilling of material from acomposite flywheel compromise the structural integrity of the compositepart, but furthermore, such machining limits the accuracy of balancingobtainable in at least the following two ways. Firstly, the accuracy ofthe balancing operation is limited by the trueness of the lathe shaftonto which the flywheel is mounted during the machining operation, andby the accuracy of the mounting of the flywheel mass to the lathe shaft.Secondly, the balancing accuracy is limited by the minimum thickness ofmaterial which can be removed in the machining/grinding/drillingprocess, which in turn may be affected by the skill of the operatorand/or (if the machine tool is computer numerically controlled) by theprecision of the CNC machine. This is made more acute, since thematerial removed from the flywheel is necessarily dense (in order tomaximise the energy storage density of the flywheel).

It is desirable therefore for a method to be found for simply andquickly balancing such a flywheel to a high degree of accuracy. It isalso desirable that the method should simultaneously prove thestructural integrity of the flywheel. Such a method would save time,production cost, capital cost, and would also increase the performanceand reliability of the flywheel.

Existing flywheels are sometimes constructed such that the rotating massof the flywheel rotates inside a chamber containing a vacuum. Operatingthe rotating mass inside a vacuum is advantageous since it reducesenergy losses due to air resistance (also known as windage). However, inorder to transfer energy into and out of the rotating flywheel mass, acoupling means is required. Some existing flywheels use a rotating shaftpassing through a rotating seal in the vacuum chamber to couple torquefrom an energy source to the flywheel energy storage means. Rotatingseals are never perfect, however, since they inevitably leak andtherefore require an environmental management system to be coupled tothe vacuum chamber in order to maintain the vacuum despite leakage.Furthermore, the seals become more “leaky” with age and as rotationalspeed increases, and also wear more quickly at higher speeds. The use ofrotating seals is therefore undesirable. The mass, volume and cost ofsuch an environmental management system is undesirable.

Magnetic couplings can be used with flywheels to transfer torque througha vacuum chamber wall, thereby obviating the need for rotating seals.However, the torque transmission capability of such magnetic couplingsusing permanent magnets has previously been found to be lacking intorque transmission capability.

This has been found to be at least partly because the magnetic fluxwhich passes between the poles of the two rotating members, for a givenmagnetic pole strength, is limited by the “air gap” between the twomembers. The air gap in fact, comprises the air gap between the outerrotating member and the vacuum wall, the vacuum wall itself, and avacuum gap between the vacuum wall and the inner rotating member. Sincethe vacuum chamber wall must be structurally strong enough to supportatmospheric pressure, its thickness is necessarily significant,resulting in a large “air gap” between the inner and outer rotatingmembers.

Existing arrangements have sought to overcome this limited torquecoupling capability by employing electromagnetic poles in order toincrease the magnetic strength and thereby increase torque couplingcapability. However, the use of electromagnetic poles requires an energyconversion, thereby reducing the efficiency of the energy storageflywheel (since the electromagnets require electrical power to operatethem, which must be sourced from the energy stored in the flywheel).Furthermore, the additional control and power electronics associatedwith electromagnetic couplings significantly increases the size, andweight of a flywheel energy storage system incorporating such anelectromagnetic coupling, thereby further reducing the energy storagedensity of such a flywheel energy storage system, both in terms of massand volume. A method of coupling energy into and out of an energystorage flywheel operating in a vacuum chamber, which is efficient interms of mass, volume and energy is therefore required.

A further problem with existing flywheels is that while the flywheelitself should be able to rotate at a high angular velocity, the driveshaft which invariably couples the flywheel to an energy source or sink(such as an engine or transmission) and associated components which areoutside of the vacuum chamber suffer losses associated with airresistance (or “windage”).

Magnetic gears use arrays of magnets (for example, permanent magnets)and stationary pole pieces to transfer torque between rotatable members,for example driveshafts. They exhibit reduced wear when compared toconventional mechanical gears. However, their torque transmissioncapability is dependent on the rotational position of the magnets withrespect to each other, and therefore varies as the shafts rotate. Forexample, when torque transfer capability is plotted on a graph againstangular position, severe peaks and troughs in the torque curve can beexhibited. This is known as “cogging” and leads to a set of undesirablecharacteristics.

Firstly, peaks and troughs in the torque curve lead to the magnetic gearhaving a variable “pull-out” torque with meshing position. That is, thetorque required before the gears will slip out of mesh varies dependingon the rotational meshing position. Therefore, such a gear set fortransmitting a given level of torque must be designed such that itsminimum torque coupling capability, as represented by one of the troughs(shown at around 20 Nm in FIG. 26) in the torque curve, is greater thanthe design torque handling figure. For this, the magnet arrays must besized appropriately larger, and this also normally results in excesstorque coupling capability at certain meshing positions, representing aninefficiency. Thus, the magnet arrays are normally sized larger thanthat which would be necessary if the torque curve more closely followedthe mean torque handling capacity, thereby increasing their cost andsize, and reducing the energy storage density of a flywheelincorporating such a magnetic gear.

Furthermore, since the angular offset between the input and outputshafts of a magnetic gear varies according to the torque applied and tothe torque coupling capacity at a given meshing position, if the torquecoupling capacity varies with meshing position then this will result ina torsional vibration in the shafts. Such a torsional vibration canreduce the life of the associated mechanical components, and/or canresult in failure and/or disengagement. This is an especially seriousproblem if the rotational speed is such that the frequency of thetorsional vibration coincides with a resonance of the mechanical system.It would therefore be advantageous if the variation between the peaksand troughs in the torque curve could be reduced or eliminated. Thiswould allow smaller, cheaper, magnet arrays to be used, since theminimum torque coupling capability would then be much closer to the meantorque coupling capability. Torsional vibration of the shafts would alsobe reduced, allowing cheaper, lighter and smaller components to be used.A flywheel energy storage system employing such smaller, cheaper andlighter components would have a higher energy storage density.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, embodiments disclosed herein relate to an apparatus forcoupling force between first and second movable members, the firstmovable member has a first array of alternating magnetic poles arrangedthereon and the second movable member has a second array of alternatingmagnetic poles arranged thereon, a magnetic flux coupling means isarranged between the first and second movable members, there is a firstspacing between the magnetic flux coupling means and the first movablemember and a second spacing between the magnetic flux coupling means andthe second movable member, and the magnetic flux coupling means isincorporated in a membrane which is impermeable to fluids and separatesthe first and second moveable members, the membrane is arranged toenclose a vacuum around a flywheel.

In another aspect, embodiments disclosed herein relate to a method ofmaking a coupler for coupling force between first and second movablemembers, the method including arranging an array of alternating magneticpoles on each of first and second members, separating the first andsecond members with a membrane which is impermeable to fluids, whereinthe membrane is arranged to enclose a vacuum around a flywheel, andincorporating magnetic flux coupling means in the membrane.

FIGURES

Embodiments of the invention will now be described with reference to thedrawings, of which:

FIG. 1 is a representation of a known flywheel;

FIG. 2 is an isometric view of an embodiment of the present invention;

FIG. 3 is a cross-sectional view of the embodiment of FIG. 2;

FIG. 4 is a view of a shaft construction;

FIG. 5 is a detailed view of a winding pattern;

FIG. 6 is a view of the winding at a shaft;

FIG. 7 is a view showing alternative winding methods at a rim;

FIG. 8 is a cross-sectional view of a flywheel incorporating a warningring;

FIG. 9 is a side view of the embodiment of FIG. 8;

FIG. 10 is a view of another embodiment incorporating a warning ring;

FIG. 11 is a side view of the embodiment of FIG. 10;

FIG. 12 is a view of a further embodiment having a warning ring;

FIG. 13 is a side view of the embodiment of FIG. 12;

FIG. 14 is a view of a yet further embodiment incorporating a warningring;

FIG. 15 is a view of a flywheel constructed inside a vacuum chamber;

FIG. 16 is a cross sectional view of the flywheel of FIG. 15;

FIG. 17 a is a view of a type of magnetic coupling;

FIG. 17 b is a view of a magnetic coupling having a coupling element;

FIG. 18 a is a view of an epicyclic magnetic gear coupling;

FIG. 18 b is a close up view of part of the coupling of FIG. 18 a;

FIGS. 19 a to 19 c are sequential views of part of the coupling of FIG.18 a as it rotates through a sequence of three positions;

FIG. 20 is a cross sectional view of a magnetic coupling such as thatshown in FIG. 18 a when incorporated in a vacuum chamber wall;

FIG. 21 is a view of a magnetic gear coupling incorporating staggeredpoles;

FIG. 22 is a view of an epicyclic magnetic gear coupling incorporatingstaggered poles;

FIG. 23 is a view of an epicyclic magnetic gear coupling incorporating aspread coupling element;

FIG. 24 is a view of a contra-rotating magnetic gear couplingincorporating staggered coupling elements and a spread coupling element;

FIG. 25 is a cross sectional view of a flywheel incorporating anepicyclic magnetic gear coupling;

FIG. 26 is a graph showing two curves of torque coupling capabilityversus gear meshing position.

DETAILED DESCRIPTION

In overview, the apparatus and method described herein relates to aflywheel energy storage device where material used in its constructionis deployed in an inertially efficient manner, and where the supportstructure is under tension, a rim comprising a mass element is held inplace on its outer surface by a winding which also passes around a drivetransfer element, rather than for example by a compressive interferencefit to its inner surface.

In other embodiments a support element can surround the rim tocounteract centrifugal forces and a torsionally compliant or resilientdrive transfer element such as shaft can be provided.

The winding may be configured in a number of ways as described below andmay also be pre-tensioned. The drive transfer element may be a shaft,which may be hollow and may be constructed from wound carbon fibre. Therim may comprise a circumferential support member (also referred to as asupport element) and a mass element mounted radially inwards of thesupport member.

In embodiments the rim may be constructed of a composite material, forexample a wound carbon fibre and resin. The mass element may be a ring,pressed or moulded into the reinforcing element. Alternatively, the masselement may comprise one or more dense elements, which may be linked asin a chain, incorporated into the rim by moulding, drilling, pressing oradhesive attachment to the inside of the reinforcement element.

The drive transfer element may be a hollow shaft for example, and thismay be formed from a wound carbon fibre composite. The composite may bewound with fibres oriented in directions arranged such that both bendingof the shaft and twisting of the shaft result in a change in the lengthof the fibres, these deformations therefore being resisted by thefibre's natural tendency to resist changes in length. The shaft maythereby be formed so that it is compliant to a twisting motion.

A warning or indicator ring can be mounted to the flywheel rim, and theflywheel can be arranged such that at least one of the warning ring andother components move, expand, contract, deform or distort relative tothe other under centrifugal force of sufficient magnitude. This canaffect rotation of the assembly, for example by unbalancing it, whichcan be monitored or detected to provide an indication of overload.

Because a warning or indicator ring is incorporated with the flywheel tobehave differently under rotation, when the flywheel reaches undesirablerotational speeds a detector can detect consequences of the differentbehaviour, for example, imbalance in the flywheel.

The indicator ring may be mounted to the flywheel with an interferencefit, and is supported either by the support member or by the drivetransfer element (for example, a shaft). The ring can be constructedfrom circumferentially wound fibre (for example, carbon fibre), or canbe another material with sufficient strength in hoop to enable it to berotated at the maximum designed flywheel speed without failing, and witha suitable stiffness as further described below. When mounted to thesupport member, the ring can be radially disposed inside or outside ofthe support member. When the ring is radially disposed inside of thesupport member, the ring has a stiffness which is greater than orsubstantially the same as that of the support member. When the ring isradially disposed outside of the support member, the ring has astiffness which is lower than or substantially the same as that of thesupport member. The support member comprises circumferentially woundfibre, for example carbon fibre. When mounted to the drive transferelement (for example, a shaft), the ring has a stiffness which is lowerthan or substantially the same as that of the drive transfer element.

The interference fit results in a pre-load between the ring and itsmounting (for example, the support member) when the flywheel is at rest.The level of preload and the relative stiffnesses of the ring and thering mounting are chosen such that when the flywheel is rotated at or inexcess of a predetermined trigger speed, the preload is substantiallyovercome by centrifugal forces, causing the ring and support member toseparate. Generally, the less stiff component will tend to stretch and“grow” more than the stiffer component. Notably, however, in the casewhere the ring and its mounting have substantially the same stiffness,the two components will nevertheless tend to separate under rotationbecause greater forces act upon the component which is at the greaterradius from the axis of rotation. The combination of radial position andmaterial stiffness can be adjusted accordingly to achieve separation atthe desired predetermined speed. The predetermined speed is chosen to belower than the speed at which flywheel failure is to be expected. Thering is fitted to its mounting by a press fit which results in anon-uniform stress distribution at the interference boundary.

Referring to FIGS. 2 and 7, in order to effectively provide a highinertia, a rim (50) including a mass element (10) comprising, forexample, a ring of relatively massive material is disposed at arelatively large radius, compared with the size of the flywheel (30),from a drive transfer element such as a shaft (60) providing a centralrotating axis (20). The mass element (10) has a high density in order toeffectively provide inertia. Suitable materials may be lead or steel forexample, although other materials could be used. The mass element (10)is subject to stress when the flywheel (30) is in rotation, this beinginduced by centrifugal forces.

An outer circumferential support member (40) is located radially outsidethe mass element. The support member (40) has a high hoop strength andis able to counteract the centrifugal forces acting on the mass element(10) when the flywheel (30) is in rotation. The support member (40) ispreferably a carbon fibre composite, wound in a circumferentialdirection so as to impart a high strength in hoop. In the embodimentshown the support member (40) is pressed onto the mass element (10) witha small interference preload such that the two are effectively joined,forming a rim (50). The preload only needs to be small since it merelyfunctions to hold the two elements together in an interference fit whenthe flywheel is stationary. Alternatively, the two may be joined by anadhesive bond or similar. The more efficient placement of mass,concentrating mass near the rim of the flywheel results in a lighterflywheel for a given energy storage capacity. Although the mass elementis shown in FIG. 2 as a continuous ring, alternatively it may beseparate ring segments, or may be discrete elements of mass. Forexample, an alternative arrangement is shown in FIG. 7 wherein the massmay be inserted or moulded into the support member (40), either as aring, or as discrete elements into receiving holes in the support member(40).

Referring to FIGS. 2, 5 and 6, a winding couples the rim (50) to theshaft (60). The winding is configured such that it consists ofsubstantially or partially radial portions (80) extending from the shaft(60) to the rim (50) and substantially axial portions (90) extendingaround the rim (50). In the embodiment shown the winding is filamentwound in a winding operation proceeding as follows: radial portions fromshaft (60) to rim (50), axial portions over the rim (50) so as to form a‘sling’, and then radial portions back from rim (50) to shaft (60), in arepeating fashion. The winding (87) may pass at least partially aroundthe circumference of the shaft (60) between some but not necessarily alliterations of the winding operation. The winding (80, 90) will stretchslightly as the rim (50) grows under centrifugal forces and will exert acounteracting force on the rim (50). Thereby, the winding (80, 90)assists the support member (40) of the rim (50) in resisting thecentrifugal forces acting on the mass element (10) and in resisting theradial growth of the rim (50). The winding (80, 90) could be made of afibre, including carbon, glass fibre, Kevlar, Zylon or nylon, or couldbe made of a metal wire in low stress applications. As a result moremassive mounting arrangements such as a central support section orspokes are not required.

In embodiments where the mass element comprises a ductile or malleablematerial, the support member (10) and the winding (80, 90) can bepre-tensioned during manufacture by the following method: The flywheelis assembled in the way herein previously described, with drive transferelement (60) and rim (50) coupled by a winding, the rim (50) comprisinga mass element (10) and an outer support member (40). No or negligiblepre-load inwardly need be applied at this stage. The flywheel is thenspun at an angular velocity sufficiently high that the centrifugalforces on the mass element (10) are sufficient to cause it to yield andsmaller than its ultimate tensile strength. As a result, the masselement (10) yields outwardly and its circumference increases. Theincrease in circumference of the mass element (10) results in a secureinterference fit between mass element (10) and support member (40),thereby stretching and pre-tensioning the support member (40) and alsostretching and pre-tensioning the winding (80, 90). The mass element(10) has a low to moderate Young's modulus, which is less than that ofthe support member (40), such that the mass element's (10) tendency todeform under centrifugal forces is greater than that of the supportmember (40). This operation results in a pre-tensioning of both thesupport member (40) and the winding (80, 90). In this way, both thesupport member (40) and the winding (80, 90) are pre-tensioned, comparedto the result of fitting the mass element (10) to the support member(40) with an interference fit before adding the winding, which wouldresult in a pre-loading of the support member (40) only. In otherembodiments the above method can be used to pre-tension the supportmember alone.

In other embodiments, a material with an extremely low Youngs's moduluscomprises the mass element (10), such as Lead. The use of a dense liquidsuch as Mercury results in a flywheel in which the mass element (10) isself-balancing. The support member (40) constrains the mass element (10)radially inside the support member (40).

Suitably ductile or malleable materials for use in comprising the masselement (10) have a large ultimate tensile strength compared with theirfirst point of yield strength, defining a sufficiently large ductileregion that the yield point of the material can be exceeded during themanufacturing operation detailed above without a risk of exceeding theultimate tensile strength of the material. A suitable ratio of yieldstrength to ultimate tensile strength would be close to 1:2. Thematerial used for the mass element (10) also has a first point of yieldwhich is sufficiently low that it may be exceeded at moderate flywheelspeeds such that failure of other parts of the flywheel is avoided, suchparts being for instance the outer support member (40) and winding (80,90). The material also has properties such that the centrifugal forcesresulting in the pre-loading process cause a sufficiently largecircumferential deformation of the mass element (10) that the resultingdeformation of the support member (40) and winding (80, 90) results in apre-load which significantly counteracts centrifugal forces acting onthe mass element (10) when it is rotating at the typical rotationalspeeds encountered during normal operation.

In embodiments where the mass element (10) is not ductile and is notpre-loaded using the above method, the ultimate tensile strength of themass element is optimally close to that of the support member (40) andthe yield strength of the mass element (10) is as close as possible tothe ultimate tensile strength of the support member (40).

Referring to FIG. 5, the angle of the winding portions (80) from shaft(60) to rim (50) may be selected to determine the characteristics oftorque transfer between shaft (60) and rim (50). The angle used may beselected between i) a tangent to the shaft circumference and ii)perpendicular to the shaft circumference. A selected angle which isclose to a perpendicular angle to the shaft (60) will enhance thecontribution made by the winding (80) to the counteraction ofcentrifugal forces acting on the mass element (40). A selected anglewhich is close to a tangent to the shaft circumference will enhance theability of the winding (80) to transfer torque between the rim (50) andthe shaft (60). A compromise angle within the range of angles above canbe selected in order to optimise the contribution made by the winding.Since the winding (80, 90) is only able to transfer torque when intension, the radial winding portions (80) can be arranged in bothclockwise (85) and anticlockwise (86) directions such that eitherclockwise (85) or anticlockwise (86) winding portions are in tension,depending on whether the flywheel is accelerating or decelerating. Yetfurther the axial position along the shaft about which the winding isarranged can be varied to vary the strength of tensile support.

Referring to FIG. 2, the number of turns of the winding (80, 90), and asa result its strength, may be varied. Likewise, the number of turns offibre in the carbon support member (40) may be varied so as to alter itsstrength. Since the reaction against the centrifugal forces acting onthe mass element (10) is a combined reaction from the support member(40) and the winding (80, 90), the relative contribution from each canbe varied by altering the number of turns in the winding (80, 90) andthe number of turns in the support member (40). In one aspect, thesupport member (40) can be removed altogether, the centrifugal forcesacting on the mass element being counteracted solely by the winding (80,90). Yet further the winding can extend continuously around the wholecircumference or can be interrupted with gaps in the circumferentialdirection between individual or groups of fibres, providing a“spoke-like” arrangement. For example, in the case where the masselement is a number of discrete elements the winding at the rim (90) canbe aligned with the discrete mass elements.

Referring to FIG. 3, the rim (50) is at least temporarily supported by acarrier portion (70) on the drive transfer element, which may be a shaft(60). The carrier portion is preferably made of a lightweight materialso as to reduce overall flywheel mass and concentrate mass at theperiphery. The carrier portion may for example be made of wood, wax,resin or other lightweight material. The carrier portion allows mountingof the rim on the drive transfer element while the winding is beingapplied during manufacture. The carrier portion may be removed orremovable after the winding has been applied to the rim and drivetransfer elements, by way of erosion, dissolution, melting orsublimation.

The winding and the carrier portion are relatively light compared withthe rim, thereby the flywheel may thus be configured with a rimcomprising a mass element such that the majority of the mass of theflywheel is near the rim where it is most inertially efficient. Thecarrier portion (70) may be glued to the shaft (60) and/or the rim (50).

Referring to FIG. 4, the shaft (60) may be solid, but is preferablyhollow so as to reduce its mass. The shaft (60) is preferably a carbonfibre composite, woven such that it is torsionally compliant and axiallystiff. The shaft could however be made of other materials such as glassfibre, steel, titanium, other metals or composites. In the case of afibre composite shaft, the weave pattern of the fibres may be altered soas to influence the degree of resistance to bending and twisting, and tofine tune the torsional compliance of the shaft. The shaft may have oneor more bearing surfaces (65) pressed or glued onto it. One or morebearing surfaces (65) may also incorporate a drive coupling (66), or aseparate drive coupling may be glued or pressed onto the shaft. Thetorsional compliance of the shaft has the effect of limiting peak torquelevels at the drive coupling and therefore allows the use of drivecouplings with lower peak torque handling capability than that ofsplined drive couplings, for example frictional or magnetic couplings.

Manufacture of the flywheel can be further understood by referring toFIGS. 3 and 6. The winding (80, 90) can be formed by a ‘wet winding’process whereby a binder is provided for example using a resin oradhesive. The fibre which forms the winding (80, 90) can be impregnatedwith a resin or adhesive and can be wound while the resin or adhesivewas still ‘wet’, that is to say that the resin or adhesive is in theuncured state. Alternatively, the support member (40) can be coated witha resin or adhesive before or during the process of forming the winding(80, 90) such that the winding (80, 90) adheres to the support member(40). Likewise, the shaft (60) can be coated with a resin or adhesiveprior to or during the winding process such that the winding (80, 90)adheres to the shaft (60). These techniques enhance the transfer oftorque between shaft (60) and rim (50). Alternatively, an interferencefit between winding (80, 90), shaft (60) and rim (50) could be used.

Referring to FIG. 7, the winding (80, 90) and the support element (40)have henceforth been described as separate wound elements. However, itwould be possible to combine both elements by, for instance,interleaving turns of the winding (80, 90) and turns of the supportelement (40). It would also be possible to first form the supportelement (40), form holes (45) through it, and then form the winding (80,90) with the winding portions (80, 90) passing through the holes (45) inthe support element (40). The shape of the support element (40) may behemispherical or parabolic in order to spread stress in the portion ofthe winding (90) which contacts the support element (40). Any smoothsectional outline shape is envisaged as being suitable.

Referring to FIGS. 2 and 5, spaces may be left between the windingportions at the rim (80, 90) such that access to the carrier portion(70) remains. The carrier portion (70) may be left in place or may beremoved by blasting, erosion, dissolution, melting or sublimation, afterthe winding (80, 90) has been formed. The carrier portion could forexample be made of ceramic, resin, wax or other suitable material toenable this operation. Removing the carrier portion (70) would result inan even lighter flywheel having an even lower proportion of inertiallyinefficient mass. With the carrier portion removed, the winding providesthe only substantial means of support for the rim on the drive transferelement.

In alternative approaches the flywheel can be constructed with thecircumferential support member providing hoop strength but the ringbeing mounted using a conventional central support section rather than awinding.

In use the flywheel may be mounted in a vehicle or any other appropriatesetting for storage of energy or other purpose such as stabilisation andcoupled or decoupled from a drive-providing or receiving component suchas a motor, engine or dynamo as appropriate via the drive transferelement.

Referring to FIGS. 8 and 9, which show a first embodiment of a flywheel(30) having a warning or indicator ring (800), it can be seen that thewarning ring (800) is mounted on the outer periphery of the supportelement (40). The warning ring (800) is mounted radially outside thesupport element (40), using an interference fit, and is typicallypressed into place. The interference fit between the warning ring (800)and the support element (40) results in a pre-load force between thesetwo components when the flywheel (30) is at rest. The assembly ofwarning ring (800) to support element (40) results in a residualnon-uniform stress between the two. The winding (80) passes around thewarning ring (800), support element (40) and mass element (10). Theflywheel is finely balanced to avoid vibration when rotating. Duringmanufacture, the balancing operation is performed after the warning ringis assembled such that it is balanced with the warning ring in place.

As shown in FIGS. 8 and 9, the winding (80) passes around the warningring (800) and support element (40) Thus, the winding tends to hold thewarning ring (800) in contact with the support element (40),counteracting the warning ring's tendency to grow away from the supportelement (40). However, by selecting the stiffnesses of the warning ring(800), winding (80) and support element (40) appropriately it ispossible to ensure that the warning ring (800) is able to move radially(i.e. grow) away from the support element (40) under centrifugal forces.In other embodiments (such as shown in FIGS. 10 and 11) the warning ring(800) is pressed onto the outside of the support element (40) andradially outside the winding (80).

In the embodiments shown in FIGS. 8 to 11, the warning ring (800) has alower Young's modulus (is less stiff) than the support element (40) suchthat in operation when the flywheel is rotated, the warning ring (800)grows radially (under centrifugal forces) a greater amount than thesupport element (40) grows, leading to separation when the centrifugalforce reaches a sufficient magnitude. In the embodiments shown in FIGS.8 and 9 where the winding passes around the warning ring (800), thestiffness of winding (80) and warning ring (800) are together low enoughsuch that the warning ring (800) and winding (80) grow more than thesupport element (40) grows when the flywheel is rotated. The warningring (800) need only be a lightweight ring with relatively low strengthcompared to the support element (40), since the warning ring (800) doesnot substantially support the mass element (10).

Expansion of the warning ring leads to a relaxing of the pre-loadbetween the warning ring (800) and the support element (40). At atrigger rotational speed or centrifugal force magnitude (predeterminedby the amount of interference fit pre-load, and the relative stiffnessesof the warning ring and the support element), the pre-load is overcomeand the warning ring (800) and support element (40) at least partiallyseparate. The separation is likely to occur non-uniformly for example,because the interference fit has a non-uniform stress distribution atthe interference boundary, leading to a movement off-centre and animbalance in the rotating mass. Furthermore, the residual non-uniformstresses between the warning ring (800) and the support element (40) areat least partially released by the movement of the warning ring (800)with respect to the support element (40). This movement causes theflywheel (which is finely balanced during manufacture) to go at leastslightly out of balance. The imbalance cause by relaxation of theresidual stresses is permanent (that is, the imbalance is permanentunless the flywheel is subsequently at least partially re-manufactured,for example by at least performing the step of re-balancing the flywheeland optionally, prior to rebalancing, performing the steps of removingand re-mounting the warning ring onto the support element such that theresidual non-uniform stress is restored, thereby restoring the capacityof the flywheel to go out of balance if the pre-load is again overcome)and can be considered to be evidence of a mechanical “fuse” having beentriggered.

The resulting imbalance causes a vibration when the flywheel is rotatingand the vibration can be detected by a vibration sensor so as to give anindication of excessive flywheel speed, the indication being separatefrom any indication derived from, for example, a flywheel speed sensor.An example of a suitable vibration sensor is a piezo-electricaccelerometer. Thus, even if the main flywheel speed sensormalfunctions, a separate and independent indication of excessiveflywheel speed is provided. Furthermore, a permanent indication resultsshowing that the flywheel has at some point been operated above itsdesign speed and thus might fail at some point in the future.

In the second embodiment shown in FIGS. 10 and 11 the warning ringpasses outside the winding (80) and its relative stiffness is selectedaccordingly to provide the same effects.

In a further embodiment, as shown in FIGS. 12 and 13, the warning ring(800) is mounted with an interference fit, radially inside the supportelement (40). The mass element (10) is interposed between the supportelement (40) and the warning ring (800) in this embodiment, but in otherembodiments can be incorporated in the support element (40) aspreviously described or the warning ring can be interposed between themass element (10) and support element (40). In these further embodimentsthe warning ring (800) has a higher Young's modulus (is stiffer) thanthe support element (40).

In operation when the flywheel is rotated, the support element (40)grows radially (under centrifugal forces) a greater amount than thewarning ring (800) grows. Similarly to the previous embodiments, thepre-load between the warning ring (800) and support element (40) isovercome by centrifugal forces, allowing the warning ring (800) to move.When the support element (40) grows radially such that the space withinit is larger than the outside diameter of the warning ring (800), thewarning ring (800) is able to move off-centre within the support element(40), leading to an imbalance. Furthermore, under influence of thenon-uniform residual stresses (residual from the press-fitting assemblyoperation during manufacture whereby the warning ring is pressed intothe centre of the support element), the warning ring (800) is caused tomove within the support element when the pre-load is overcome bycentrifugal forces, thereby causing the flywheel to go permanently outof balance, causing vibration. As previously described, vibration can bedetected by a sensor and used as a warning indication.

In a yet further embodiment, the warning ring (800) is press-fitted tothe drive transfer element (which is, for example, a shaft) with aninterference fit which results in a pre-load. As before, the flywheel isfinely balanced. The warning ring (800) is less stiff than the shaft(60) and grows radially more than the shaft grows when the flywheelrotates. At a predetermined speed, the pre-load is overcome, allowingthe warning ring (800) to move on the shaft which causes an imbalancewhich can be detected prior to mechanical failure.

The deliberate production of an imbalance when a flywheel speed exceedsa trigger speed, and detection of a vibration caused thereby, asdescribed above, provides a warning that the flywheel is being operatedor has been operated at above its maximum safe operating speed. Thiswarning can be determined separately from a primary flywheel speedmonitoring system and thus provides a fail-safe second indication ofexcessive flywheel speed in the event that the primary speed monitoringsystem fails. It will be noted that detection of overload can betriggered by setting at the detector the level of imbalance signifyingoverload, or by modifying the relative properties of the warning ringand/or other rim components, or any combination thereof. The system canbe calibrated to indicate excessive speed when all or part of thewarning ring detaches, or when relative movement/dimension change issufficient to create a detectable or threshold-surpassing imbalance.

The embodiments where the warning ring (800) is enclosed by the winding(80) have the advantage that should the flywheel be operated at a speedhigher than the trigger speed, with the result that the warning ringbecomes loosened from the support element (40), the warning ring (800)is contained within the winding (80) and there is no danger of thewarning ring (800) becoming completely detached.

It will be seen that, as a result of the configuration described above,a stronger, safer and more efficient flywheel can be provided.

A method of balancing such a flywheel will now be described. Referringto FIG. 15, a flywheel (30) can be placed inside a vacuum chamber(1550). Operating a flywheel in a vacuum is advantageous since itreduces frictional losses/overheating associated with our resistance (or“windage”). The flywheel rim (50) inevitably has surface irregularities(1630) which result from inadequacies in the balancing operation whichis performed on the flywheel during its manufacture, and/or the methodsused to construct the flywheel. It has been found, as previouslymentioned that these irregularities result in an imperfect rotationalbalance of the flywheel (30).

In the embodiment shown, the flywheel is supported in the vacuum chamber(1550) by bearings such that the flywheel is able to rotate inside thevacuum chamber. The vacuum chamber is a sealed chamber capable ofwithstanding forces exerted by a pressure difference between atmosphericpressure and the pressure inside the vacuum chamber. The thickness ofthe vacuum chamber wall is made sufficient to give it enough strength tosupport atmospheric pressure against the vacuum inside the chamber. Thevacuum chamber incorporates at least one of a gas inlet (1520) and a gasoutlet (1510). Optionally the gas inlet and gas outlet are combined as asingle port. Each of the gas inlet and gas outlets communicate with theinterior of the vacuum chamber.

A coupling (1566, 1567) is comprised of first and second members and isarranged to couple torque between a rotatable driveshaft (1570) and theflywheel shaft (60). A first member (1566) is coupled to the flywheelshaft (60), and a second member (1567) is coupled to the drive shaft(1570). The flywheel shaft (60) is supported on bearings and isconnected to the flywheel rim (50) by means such as already described.The flywheel rim (50) is comprised of a composite material which ispreferably finely balanced by machining, drilling, or grinding duringmanufacture. The flywheel rim (50) in this embodiment is a compositeconstructed using a circumferentially wound fibre and resin aspreviously described. The rim is coupled to the shaft (60) by radialfibres such that torque can be transmitted from the flywheel shaft (60)to the flywheel rim (50).

The driveshaft (1570) is supported by bearings outside of the vacuumchamber and is rotatable. The flywheel shaft (60) and the driveshaft(1570) are supported such that the two coupling members (1566, 1567) arearranged in close proximity with the wall of the vacuum housing (1550)arranged therebetween. The members are arranged so as to minimise the“air gap” between the coupling members (1566, 1567) and the vacuumchamber wall (1550). The term “air gap” is used to describe in generalthe total gap between the two members of the coupling (1566, 1567). Thevacuum chamber can be constructed in any commonly known manner, e.g.casting, machining etc.

Referring to FIG. 16, a valve (1610, 1620) is incorporated in orattached to the inlet port (1520 a) and the outlet port (1510 a) of thevacuum chamber (1551). In operation, the valves can be opened or closedso as to selectively seal the inside of the vacuum chamber (1550) fromthe atmosphere, or allow communication between the interior of thevacuum chamber (1551) and the atmosphere. The outlet port can beConnected in use to a vacuum pump (not shown).

In use, the vacuum chamber is sealed from the atmosphere by closing theinlet valve (1620) so as to isolate the inlet port (1520 a) which is atatmospheric pressure from the interior of the vacuum chamber (1551). Theoutlet valve is typically connected to a vacuum pump capable ofproducing a high, or hard, vacuum. The outlet valve (1610) is opened soas to allow the outlet port (1510 a) which is connected to the vacuumpump to communicate with the interior of the vacuum chamber (1551). Thevacuum pump is then run until the vacuum chamber (1551) contains a hardvacuum. Preferably, this vacuum is better than 1 mbar, typically 10⁻²mbar. The flywheel (30) is then rotated by the application of torquefrom the driveshaft (1570) via the coupling (1566, 1567) to the flywheelshaft (60). This in turn rotates the flywheel rim (50). The flywheel isrotated at a speed such that the rim (50) surface is travelling at aspeed which is in excess of the speed of sound (Mach 1). The flywheelsurface speed is also referred to as the “tip speed”. Prior to thisoperation, as previously stated, the flywheel will have been balanced bymechanical operations such as grinding, drilling, or machining, to ashigh a degree as practical (within cost constraints) or at least as highas necessary to enable the flywheel to be rotated at such a speedwithout danger of mechanical failure.

Next, while the flywheel is rotating at a perimeter speed of at leastMach 1, the inlet valve (1620) is opened so as to allow an amount of gasto enter the vacuum chamber. This gas is preferably a non-reactive gassuch as nitrogen, and is preferably a dry gas, that is it does notcontain significant amounts of water vapour, so as to avoid introducinghumidity into the assembly. If the gas is to be anything other thanplain air, the inlet port would first need to be connected to a suitablesupply of said gas. The amount of gas admitted is sufficient so as tosubstantially reduce the vacuum to a pressure substantially higher than10⁻¹ mbar, for example as high as 0.5 bar. I bar works well. The rate ofgas entry has been found to be non-critical.

Upon admission of the gas into the vacuum chamber (1551), shock wavesare set up in the gas between the surface irregularities (1630) and thewall of the vacuum chamber (1551). The shockwaves, and also frictionbetween the surface irregularities (1630) and the gas, act to vapourise,melt, sublimate, erode or abrade the surface irregularities (1630) so asto reduce their size and thereby improve the balance of the flywheel(30) to a higher degree of balance then that obtainable by mechanicalmachining, drilling, or grinding alone.

Furthermore, a flywheel is normally designed for a maximum safeoperating speed. During manufacture, such a flywheel must be proved towithstand the maximum rotational speed it is designed for. This istypically done by rotating the flywheel at a speed equal to the squareroot of 2 multiplied by the design speed. Especially in compositeflywheel construction types, it can be safely assured that if theflywheel survives rotation at this higher speed then it will alwayssurvive operation at the design speed for its lifetime duration. It isfurther advantageous to combine this proving operation with thebalancing operation described above.

The rate of gas admission has not been found to be critical. After gasadmission, the flywheel is allowed to slow to rest, but continues torotate at a tip speed of at least Mach 1 for around 10 to 60 seconds,typically 15 seconds. This has been found to be long enough to removethe surface irregularities while avoiding overheating the flywheel rim.The time taken for the flywheel to come to rest was in one embodimentapproximately 3 minutes. The gas density is not homogenised throughoutthe vacuum chamber. A non-reactive gas is preferred so as to avoidreaction of the gas with the flywheel components. When the flywheel isrotated at a tip speed in excess of Mach 1, the supersonic shock waveproduced causes a far better balancing effect than if subsonic flywheelspeeds are used.

In embodiments it is desirable to use magnetic coupling for example toavoid the need for rotating seals isolating a vacuum. FIG. 17 a, shows aprior art magnetic coupling for coupling two rotatable shafts (60,1570). Each shaft is coupled to a coupling member (1766, 1767)comprising an array of alternating magnetic poles. The two arrays arearranged close to each other, such that magnetic flux can pass from oneto the other via an air gap (which is preferably as small as possible).Torque can thus be usefully transferred from one shaft to the other.

This can be particularly useful in a flywheel application since the twocoupling elements (1766, 1767) are not required to touch in order totransmit torque therebetween. The wall of a vacuum chamber (1550) can beplaced between the coupling elements (1766, 1767), thereby allowingtorque to be coupled between a flywheel (30) inside a vacuum chamber(1550) and a driveshaft (1570) outside of the vacuum chamber. Thisallows the vacuum chamber to be sealed without the use of rotatingseals, as described above. Running a flywheel in a vacuum is usefulsince it avoids air resistance (“windage”) related losses. Thus becomeseven more important if the flywheel runs at supersonic speeds. Thevacuum avoids supersonic shockwaves and/or overheating due to frictionwith air. However, since the vacuum chamber wall thickness forms part ofthe air gap between the coupling elements (1766, 1767), the ease withwhich magnetic flux is able to pass from one coupling element to theother is reduced, therefore the flux density is reduced, and the torquecoupling capability is resultingly reduced. The following embodimentssolve this problem.

Referring to FIG. 17 b, a coupling element (1730) is placed between thecoupling elements (1768, 1769). The magnetic coupling element (1730) hasa high relative magnetic permeability (in excess of 400) and thereforein operation magnetic flux passes easily through it, from the poles(1711, 1721) of the first member (1768) to the poles (1741, 1751) of thesecond member (1769) and vice versa. The coupling element is effectively“transparent” to the magnetic field. The coupling element (1730) is of amaterial having a high magnetic permeability, for example soft iron. Thecoupling element (1730) should also have as high as possible electricalresistance, so as to reduce induced eddy currents and the losses due toresistive heating associated therewith. Although a single couplingelement (1730) is shown for clarity, several coupling elements arearranged between the first and second members (1768, 1769). Sufficientcoupling members are present, so as to span at least two north-southpoles pairs of the member (1768, 1769) having the widest spaced apartpoles (1711, 1721, 1741, 1751). The space between coupling elements hasa much lower magnetic permeability than the coupling elements, anexample material is plastic. When arranged thus, in use, magnetic fluxis coupled via each coupling element (1730) from the poles of eachmember (1768, 1769) and thereby torque is coupled between the first andsecond members (1768, 1769). Notably, in use, contrary to FIG. 17 a inwhich the first and second members contra rotate, the members of FIG. 17b rotate in the same direction. The surfaces of the first and secondmembers of FIG. 17 b actually pass in opposite directions relative toeach other.

When the coupling of FIG. 17 b is incorporated in a vacuum enclosedflywheel application, the coupling elements (1730) are incorporated inthe vacuum chamber (1551) wall. This has the advantage that the vacuumchamber wall thickness does not contribute to the total “air gap”between the poles of the first and second members (1768, 1769). Thetotal “air gap” is made up of the gap between the surface of the firstmember poles and the surface of the vacuum chamber wall, plus the vacuumchamber wall thickness, plus the gap between the vacuum chamber wall andthe second member poles, minus the thickness of the coupling element.Thus, the coupling element significantly reduces the total air gap. Asmaller air gap presents less resistance to magnetic flux, therebyallowing a greater flux density between the poles of the first andsecond members in use, and therefore a greater torque couplingcapability. This is highly advantageous over conventional arrangementsusing magnetic couplings through a vacuum chamber wall.

The magnetic poles (1711, 1721, 1741, 1751) are rare earth magnets,since these exhibit high field densities for a given volume of magneticmaterial. The magnets are smaller lighter, more compact, and able totransmit greater torque. Rare earth magnets have also been found to begood at withstanding compressive forces and are therefore suitable forplacing on the inner circumference of a flywheel which rotates at highspeed.

Referring now to FIG. 18 a, a concentric embodiment of the magneticcoupling shown in FIG. 17 b is illustrated. FIG. 18 a is across-sectional view showing the first member (1770) concentricallyoutside the second member (1780), and the vacuum housing (1550)concentrically therebetween. Incorporated in the vacuum housing (1552)are the coupling elements (1731). In this concentric embodiment, thefirst and second members contra-rotate. In common with the embodiment inFIG. 17 b, the surfaces of the members move in opposite directionsrelative to each other.

The number of coupling elements required, for an evenly spaceddistribution around the circumference of the vacuum housing between thefirst and second members, is equal to the number of north/south polepairs of the first member (1770) added to the number of north/south polepairs of the second member (1780). The coupling elements can be confinedto particular regions around the circumference of the vacuum housing, orcan be distributed evenly around its circumference. In the case wherethe coupling elements are confined to particular regions, the couplingelements (1731) are spaced with respect to each other as if the fullnumber of coupling elements were equally spaced around the vacuumchamber wall, except that some elements are omitted. The positioning isideally chosen such that coupling elements are positioned symmetricallyaround the vacuum chamber wall circumference, so as to avoid net forcesresulting. A minimum number of coupling elements required is that whichwill span two pairs of north/south pole pairs of whichever of the firstand second members have the greater pole spacing. This minimum numberguarantees that torque can be transferred between the members and thatthe relative directions of rotation of the first and second member iswell defined.

Backing iron (1890) is arranged on the side of the poles facing awayfrom the coupling elements so as to aid the transmission of magneticflux between the mutual pole pairs of each one of the first and secondmembers. Further, the backing iron aids the longevity of the permanentmagnets.

Such a concentric magnetic geared coupling can be constructed usingstandard machining techniques and using the materials as described forthe embodiment shown in FIG. 17 b.

The first and second members (1770, 1780) can have the same number ofnorth/south pole pairs, or can have a different number of north/southpairs. In the shown embodiment, the second member has a lower number ofnorth/south pole pairs than the first member. In operation, when thefirst member (1770), having a number of north/south pole pairs m, isrotated in a anticlockwise direction, the second member (1780), having anumber of north/south pole pairs n, rotates in a clockwise direction.The second member rotates at a speed relative to the rotational speed ofthe first member multiplied by a factor: n divided by m. FIG. 18 b showsthe lines of magnetic flux (1880) which pass between the poles of thefirst and second members, via the coupling elements (1730) which areembedded in the vacuum chamber wall (1552).

FIGS. 19 a to 19 c show a sequence of a rotation of the first and secondmembers through three positions. FIG. 19 a shows the lines of fluxbetween the poles of the first and second members in a first position.FIG. 19 b shows the top member having rotated slightly in a clockwisedirection, and the bottom member having rotated slightly in an anticlockwise direction. The lines of flux have accordingly moved position,and in particular a line of flux (1880) has stretched. FIG. 19 c shows afurther rotation of the top member in a clockwise direction and of thelower member in an anti clockwise direction. The line of flux (1880) hasnow stretched so far that it has broken and flux has switched to passinstead via the left most coupling element (1895) to form a new line offlux (1890). The torque transferred from the first member to the secondmember is equal to the rate of change of flux as the lines of fluxswitch from one route to another route in this way.

A further advantage from the use of rare earth magnets results fromtheir high flux density per unit size, particularly when used in thisway, since it is possible to arrange a large number of pole pairs aroundthe circumference of the first and/or second members and therebyincrease the rate of change of flux and thereby increase the torquecoupling capability.

Also, due to the relatively small size of rare earth magnets for a givenstrength, it possible to have a large ratio between the number of polepairs on the first member and the number of pole pairs on the secondmember, since many magnets can be packed into a small size therebydelivering a high gear ratio in a compact size. This has the advantageparticularly in flywheel applications employing a vacuum chamber (1550)in that the driveshaft and associated components which run in air areable to be run at a lower speed, thereby reducing losses associated withwindage and air resistance, while the flywheel inside the vacuum chamber(1550) is geared by the magnetic coupling to run at a higher speed, soas to increase the energy storage density of the flywheel.

Existing systems employ a gear box to allow the flywheel inside thevacuum chamber to rotate at a high speed while the drive shaft to theenergy source/sync is able to rotate in air at a slow angular velocity.However, gear boxes suffer frictional losses and increase the cost,complexity and size of the energy storage system.

FIG. 20 shows an embodiment of a flywheel (30) having a rim (containingthe majority of the mass) (50), mounted on a shaft (60), coupled to afirst element (1771) and housed within a vacuum chamber (1553), thevacuum chamber incorporating coupling elements (1732). In thisembodiment, the drive shaft (1570) is coupled to the second element(1781). The driveshaft and the flywheel shaft are supported on bearings(2010). Each of the first and second members (1771, 1781) has poles(1713, 1723, 1743, 1753). Thus, the flywheel can be driven in a vacuumat high speed by the driveshaft which is coupled to the flywheel via thefirst and second members and the magnetic poles. Due to the gearingeffect brought about by the unequal numbers of pole pairs on the firstand second members, the drive shaft, which runs in air, is able to runat a lower speed thereby reducing “windage”, or air resistance relatedlosses.

Furthermore, the coupling elements (1732) reduce the air gap between themagnetic poles and enable permanent magnets to be used to couple a highlevel of torque between the first and second elements, avoiding the needfor an energy conversion, as would be required for example ifelectromagnets were used. By using the coupling elements (1732)electromagnets are not required since the more efficient arrangementallows the more limited field strength of permanent magnets to besufficient.

According to the approach described, the use of rotating seals iscompletely eliminated, thereby eliminating the need for environmentalmanagement apparatus to maintain the vacuum inside the vacuum chamber(1553). The vacuum inside the vacuum chamber can remain thereindefinitely since the chamber is completely sealed, using no rotatingseals which can leak. The removal of the associated environmentalmanagement equipment (for example a vacuum pump, lubrication pump,associated pipe work and systems, control systems/electronics) furtherreduces the flywheel storage system weight and size and increases theenergy storage density. Furthermore, reliability of this simpler systemis accordingly improved and cost is reduced. Thereby a highly efficientflywheel energy storage device is provided.

The coupling also has the advantage that if an over-torque conditionoccurs, the coupling harmlessly slips while the over-torque conditionexists, and then later resumes normal function with no adverse effects.Furthermore, due to Earnshaw's Law, only torsional energy is transferredvia the coupling, therefore the coupling gives axial and radialisolation in respect of vibration. In an alternative embodiment, thecoupling elements could be supported in a third member which is drivenby a shaft or could drive a shaft, so as to provide further gearingratios.

The removal of rotating seals also allows the flywheel to rotate at afaster speed than would otherwise be possible due to degradation ratesof the seals (which become worse as rotation speed increases), furtherincreasing the energy storage density. Parasitic losses due to shear inthe seal lubrication fluid (which is a necessary feature of rotatingseals) will also be reduced by removal of the seals.

As previously discussed, and referring to FIG. 17 b, magnetic gears canexhibit a variable torque coupling capability with rotational meshingposition of the first and second members (1766, 1767). This has beenfound to be a result of magnetic flux (as shown in FIG. 19 c) switchingfrom a first path (1880) to a second path (1890) as the first and secondmembers (1766, 1767) move past each other. A further cause of variationin the torque coupling capability of a magnetic gear coupling is due tothe varying magnetic flux path lengths (shown in the sequence of FIGS.19 a to 19 c) as the first and second members (1766, 1767) move pasteach other. A longer magnetic flux pass experiences greater magneticreluctance, thereby reducing magnetic flux density and, as the torque isproportional to the rate of change of flux, reducing the torque couplingcapability of the magnetic gear at that angular meshing position islikewise reduced.

Following now to FIG. 26, the variation of torque coupling capabilityfor a particular physical implementation with respect to the angle of aninput shaft can be seen as the curve which exhibits large exclusions oftorque coupling capability (between approximately 20 Nm and 50 Nm).

It has been found that variation of torque coupling capability withmeshing angle (or “cogging”) can be reduced by splitting each magneticpole of a member, into “split parts” (2110, 2111, 2112, 2113, 2120,2121, 2122, 2123). The split parts are arranged in the direction ofmotion so as to form split arrays. The split arrays are arranged side byside along an axis orthogonal to the direction of motion, as shown inFIG. 21. Each split array is offset in the direction of motion withrespect to another split array, such that a spread of relative positionsis covered. The spread of positions should cover approximately at leastthe distance of a north-south pole pair of the member having the widestpole spacing. Since the relative positions of the split arrays arespread (or “staggered”) over a range of positions, it is not possiblefor a pole of each of the split arrays to each simultaneously aligncompletely with a coupling element and with a pole of the other member,thus “complete alignment” is prevented. Thereby, by splitting andstaggering poles of one or both members, and/or by splitting thecoupling element and staggering the positions of each split couplingelement part, complete alignment of the members and/or the coupleelements can be prevented.

In the embodiment shown in FIG. 21 which has first and second members1772 and 1782 (with the second member having poles 1744), there are foursplit arrays on the first member (1766). The result of this arrangement,which prevents complete alignment, is that, referring back to FIGS. 19 ato 19 c, the position (an angular position in this embodiment) at whichflux lines (1880, 1890) switch from one coupling element to anothercoupling element, or from one split pole to another split pole, variesbetween each split array. If, as in the embodiment shown in FIG. 21,there are four split arrays, and those split arrays are offset in thedirection of motion so as to prevent complete alignment of the poles andcoupling elements (rotationally offset in this embodiment), then for asmall movement (that would otherwise have caused a transition in thewhole field if complete alignment was allowed) there will now be onlyone fraction of the flux shown switching (one quarter in thisembodiment). However, in this embodiment there will be four times asmany such transitions for a particular movement distance of the assembly(e.g. a full rotation). The torque transfer for that movement is thus intotal the same, but is delivered more continuously leading to lower“cogging”. For clarity, only a single coupling member is shown in FIG.17 b. As shown in FIG. 21, this coupling member can also be split intocoupling parts (2130) to (2133). Splitting the coupling member in thisway reduces the interaction between the split arrays of the members, butis not necessary for a reduction in “cogging” to be achieved.

Referring now to FIG. 22, which has first and second members (1773 and1783) (with the second member having poles (1745, 1754)), an epicyclicembodiment of the invention is shown, this time having a first memberwith a first array of magnetic poles (in pairs) arranged on it in thedirection of motion. A coupling element (2130) is arrangedconcentrically between the first (rotatable) member and a second(rotatable) member. The coupling element is also split, into multiplecoupling parts (again, three coupling parts in this embodiment). Asingle coupling element is shown for clarity but a plurality of couplingelements is employed, forming a barrel concentrically around the firstmember. The second member is arranged concentrically outside the firstmember and the coupling elements. The second member has on its innercircumference a second array of magnetic poles, in north-south pairs,arranged in the direction of motion. The second array of poles is splitinto multiple split arrays (three split arrays in this embodiment),arranged side by side along the axis of rotation (which is orthogonal tothe direction of motion). In use, the first and second memberscontra-rotate. If one member is rotated, magnetic flux couples betweenthe poles of the first array and the poles of the second array, throughthe coupling element, and the other member is caused to contra-rotateand vice-versa.

It will also be appreciated from FIGS. 21 and 22 that, instead of, or inaddition to the splitting of first and/or second pole arrays (into splitarrays) along their axial length (the axis is orthogonal to thedirection of relative motion), and the offsetting in the direction ofmotion of each split array, each coupling element can optionally,alternatively or also, be split into coupling parts along its axiallength (2130, 2131, 2132) as shown, and these coupling parts canaccordingly also be offset. One, or a combination of these features canbe incorporated so as to diversify the positions at which magnetic fluxswitches from one path to another path as shown in FIG. 19 c. Thisstrategy may be referred to as staggering the poles, or staggering thecoupling elements. Staggering the poles and/or coupling elements resultsin a reduction of the variation of torque coupling capability whenplotted against position. This is shown in FIG. 26 as the curve whichexhibits a relatively small variation of torque coupling capability,(around 25 to 35 Nm). This represents an improvement in performance overconventional magnetic gear couplings, for the following reasons.

The minimum torque coupling capability of the improved magnetic gear isgreater and does not fall below 25 Nm, shown in FIG. 26. (In contrast,the prior art magnetic gear torque coupling capability falls at someangular meshing positions to a figure of less than 20 Nm). Accordingly,for a given design torque capability, the size of the magnets used inthe improved magnetic gear can accordingly be reduced in size whilestill delivering the torque coupling capability. The reduction invariation of torque coupling capability thereby allows such an improvedmagnetic gear to be designed with smaller, lighter and cheaper magnets.

A further advantage of the improved magnetic gear described herein isthat since the torque coupling capability has less variation, in use,when a torque is applied to the improved magnetic gear coupling, theresultant angular offset or “slippage” (being proportional to the torqueapplied and the torque coupling capability), is more constant than thatwhich would result in a prior art magnetic gear coupling. Thereby,torsional vibrations caused by this variation are reduced. The reducedtorsional vibrations are less likely to cause severe resonance whichmight damage components, require component strength to be uprated withassociated cost implications, or cause the coupling to slip out of meshand lose alignment.

A further embodiment is shown in FIG. 23 which has poles (1714, 1724,1746, 1756), whereby the coupling element (2330) follows a sinusoidalpath along an axis orthogonal to the direction of movement of the firstand second members (in this embodiment, along the axis of rotation offirst and second members) such that its position in the direction ofmotion of the first and second members varies along the axis. The shapeof the coupling element is symmetrical between its ends, along the axisso as to balance the axial forces resulting and thereby cancel them.Thereby, the position at which magnetic lines of flux switch position,as shown in FIG. 19 c, varies with axial position. Again, only a singlecoupling element (2330) is shown in figure for clarity. However,multiple coupling elements will normally be employed as describedearlier.

Furthermore, although FIGS. 18 a to 23 generally show rotatingembodiments, with the first and second members either alongside eachother or concentric with each other, as shown in FIG. 24, an end-onalignment of first and second members (1774 and 1784) is also possible.In such an end-on embodiment, the coupling element (2430) can either becurved, or can be split into parts (2431) to (2436) which are staggered,and the coupling element and/or the poles of the first and secondmembers can also be split, this time rather than being split along theaxis of rotation, they are split in a radial direction.

Furthermore referring to FIGS. 22 and 23, one or both of the first andsecond members could be unrolled so as to form a planar surface. Such anembodiment would resemble a rack and pinion, or a pair of tracksslidable over each other, with the coupling element being disposedtherebetween. In such embodiments, the first and/or second membersand/or the coupling elements would be staggered in a direction which isorthogonal to the direction of movement and parallel to a surfacebetween the members.

FIG. 25 shows a cross-sectional view of a practical embodiment which hasfirst and second members (1775 and 1785) and wherein a drive-shaft(1570) is coupled to the second member (1785) which has magnetic poles(1748) arranged in an array around its circumference. The poles (1748)of the second member (1785) are split in a direction parallel to theaxis of rotation (orthogonal to the direction of rotation) into multiplesplit parts (eight in this embodiment). This results in multiple splitarrays of split poles arranged on the second member. In this embodimentthere are eight split arrays, each arranged circumferentially around thesecond member, and each split array side by side along the axis ofrotation. Each split array is positionally offset with respect toanother. In this embodiment, each split array is rotated with respect tothe other slightly so as to span a spread of angles at least equal tothe distance between north-south pairs of poles of the member. Backingiron pieces (2580) are arranged concentrically between the second member(1767) and the magnetic split poles (1748).

Concentrically outside the second member and its pole arrays is a wallof a vacuum chamber (1554) which also extends around the axial ends ofthe device, thereby forming a toroidal shape, and incorporates in itsinner circumferential wall coupling elements (2130, 2131). This allowsvery efficient packaging of the flywheel, ease of manufacture andsealing. The coupling elements are located concentrically between thefirst and second members and are arranged in an array forming a barrelconcentrically around the second member and inside the first member.Each of the coupling elements are also split along their axial length toform multiple coupling parts per coupling element (eight coupling partsper coupling element in this embodiment). Thus, the barrel formed by thecoupling elements is split into rings and each ring is preferably offset(rotationally offset in this embodiment) from another of the rings.Alternatively, the coupling elements, instead of being split, can beshaped so as to still lie within the barrel of the inner vacuum chamberwall, but to vary their position in the direction of motion as thelength of the barrel is traversed, for example in a chevron or sinewavepattern.

Concentrically further outside, the poles (2110, 2111) of the firstmember (1775) are supported inside a composite flywheel rim (50) withbacking iron pieces (2590) interposed between the rim and the poles.Again, each of the poles (2110, 2111) of the first member are splitalong the axial length of the device into multiple split poles per pole(eight in this embodiment). The split poles are arranged around theinternal surface of the rim (50) to form split arrays (eight in thisembodiment). The first and second members (1775, 1785) are supported onbearings (2010) such that they are able to rotate. The first member(1766) is thereby able to rotate inside the vacuum chamber (1554), and asecond member is able to rotate concentrically inside the vacuum chamberbut outside of the vacuum (e.g. in air), and separated from the firstmember by the vacuum chamber (1554) wall.

In use the vacuum chamber preferably contains a hard vacuum. Althoughnot shown, the first and second members have different numbers ofnorth/south pole pairs arranged radially around them, such that a gearratio results between them. In use, this allows the second member (1785)(which operates in air) to rotate at a relatively lower speed than thefirst member (1775) which is operated in the vacuum. Thereby, lossesassociated with air resistance (or windage) when the second memberrotates are reduced. Also, the use of supersonic speeds for the firstmember and flywheel components is enabled by use of a vacuum to housethe flywheel, since supersonic shock and frictional overheating areavoided.

The vacuum chamber (1554) has no rotating seals and is therefore able tobe completely sealed without leakage (which is unavoidable when rotatingseals are used, and is worse at higher rotational speeds), therebyobviating the need for equipment associated with maintaining the vacuum,such as a vacuum pump, control electronics, pipe work etc. Removal ofrotating seals also allows higher rotational flywheel speeds, and lowerlosses due to elimination of drag. Thereby the energy storage density ofthe flywheel is increased and the associated cost of such a flywheel isreduced. Reliability is also improved due to the increased simplicity ofthis arrangement, and due to the elimination of rotating seals whichwear in use (and wear especially rapidly at high rotational speeds).

Furthermore, the “anti-cogging” features incorporated in thisembodiment, as previously described, allow the use of smaller permanentmagnets (due to the minimum torque coupling capability being closer tothe mean torque coupling capability) with associated advantages of lowercost and weight, thereby increasing the energy storage density of theflywheel. Smaller magnets also enable a higher gearing ratio to beproduced since a greater number of north/south pole pairs can be packedinto a flywheel of a given size. This higher gearing ratio furtherreduces losses associated with air resistance or windage, on the airside of the device, further increasing efficiency of the flywheel andits energy storage density. A further advantage of the anti-coggingfeatures previously described is an improvement in noise vibration andharshness, and extended service life of components due to the reductionin torsional vibration brought about by these features. This will alsoallow components to be re-specified so as to use cheaper material, orless material, thereby bringing about cost and/or weight advantages.Manufacturing efficiencies may also be gained from the ability to usematerials which would not have withstood torsional vibrations, but whichare easier to machine or process during manufacturing.

It will be seen that as a result of the features described above, astronger safer, lighter, more efficient and more effective flywheel canbe provided for energy storage.

The invention claimed is:
 1. Apparatus for coupling force between firstand second movable members, wherein the first movable member has a firstarray of alternating magnetic poles arranged thereon and the secondmovable member has a second array of alternating magnetic poles arrangedthereon, wherein: magnetic flux coupling means is arranged between thefirst and second movable members, wherein there is a first spacingbetween the magnetic flux coupling means and the first movable memberand a second spacing between the magnetic flux coupling means and thesecond movable member, wherein the magnetic flux coupling meanscomprises one or more coupling elements; and a vacuum chamber containsone of the first and second movable members, the vacuum chamber having avacuum chamber wall, wherein the magnetic flux coupling means isincorporated in the vacuum chamber wall which separates the first andsecond movable members, wherein the vacuum chamber is arranged toenclose a vacuum around a flywheel and wherein each coupling element issegmented orthogonally to a direction of relative motion between themembers.
 2. The apparatus of claim 1 in which one of the first andsecond members is coupled to the flywheel.
 3. The apparatus of claim 1in which in operation the first member has a first velocity and secondmember has a second velocity, wherein the first velocity of the firstmember is equal to the second velocity of the second member multipliedby a multiplier, the multiplier being proportional to the ratio of thenumber of magnetic pole pairs of the first and second members.
 4. Theapparatus of claim 1 in which the first and second members arerotatable, and the first and second spacings are angular spacings. 5.The apparatus of claim 4 in which the first array is arranged inside thesecond array.
 6. The apparatus of claim 4 in which the first array isarranged opposite the second array.
 7. The apparatus of claim 4 in whichthe first array is arranged alongside the second array.
 8. The apparatusof claim 1 wherein the one or more coupling elements include materialhaving a relative permeability greater than about
 400. 9. The apparatusof claim 1 wherein each coupling element has substantially the samethickness as the vacuum chamber wall.
 10. The apparatus of claim 1wherein the first and second movable members are relatively rotatableabout an axis of rotation and each coupling element havs a width thatincreases with radius from the axis of rotation.
 11. The apparatus ofclaim 1 wherein the number of coupling elements is sufficient to spantwo pole- pairs of the member having the greatest pole spacing.
 12. Theapparatus of claim 1 in which the poles of each array are substantiallyequally spaced with respect to other poles of the same array in adirection of motion between the members.
 13. The apparatus of claim 1 inwhich the poles of the first and second members are arranged with aspacing therebetween in a direction of relative motion between themembers, wherein the coupling means provides a region of relatively highmagnetic flux density between a pole of the first member and a pole ofthe second member, further in which at least one of the first array, thesecond array, and the coupling means are arranged such that thealignment of the region relative to at least one of the first member,the second member and the coupling means, varies along a line betweenthe members and orthogonal to the direction of motion.
 14. Apparatus forcoupling force between first and second movable members, wherein thefirst movable member has a first array of magnetic poles arrangedthereon and the second movable member has a second array of magneticpoles arranged thereon, wherein: magnetic flux coupling means isarranged between the first and second movable members, wherein there isa first spacing between the magnetic flux coupling means and the firstmovable member and a second spacing between the magnetic flux couplingmeans and the second movable member; and a chamber containing one of thefirst and second movable members, the chamber having a chamber wall,wherein the magnetic flux coupling means is incorporated in the chamberwall which separates the first and second movable members, wherein themagnetic flux coupling means comprises one or more coupling elements andeach coupling element varies in alignment with the poles of the firstand second members in the direction of motion, along a line orthogonalto the direction of motion.
 15. Apparatus for coupling force, theapparatus comprising a first rotatable member comprising a first arrayof magnetic poles and a second rotatable member comprising a secondarray of magnetic poles, wherein the first and second rotatable membersare arranged to contra rotate; a chamber containing one of the first andsecond rotatable members and having a chamber wall which separates thefirst and second rotatable members; and a magnetic flux couplingarranged between the first and second rotatable members, wherein themagnetic flux coupling comprises a plurality of magnetic flux couplingelements incorporated in the chamber wall.
 16. The apparatus of claim15, wherein each of the plurality of magnetic flux coupling elementvaries in alignment with the poles of the first and second members inthe direction of motion, along a line orthogonal to the direction ofmotion.